Show the font metrics

[masakudamatsu]

TODO: Start from the code deleted when we remove the direct entry of font metrics by the user (see the first commit you see below).

AIM After the user opens a font file (or clicks the Start Demo button), show the font metrics so that they make sense.

• Otherwise it's not clear whether the font file does have proper values for all the properties needed to render sample paragraphs and generate CSS code.
• Arial Black, for example, doesn't produce vertical space between paragraphs. Maybe the cap-height data is missing.
• DIN Next, for example, line spacing appears too wide.

HOW We need to render the font and illustrate what each parameter means.

[masakudamatsu]

For the time being, the extracted values of font metrics are shown in the console.